On the influencing factors of acoustic testing
When testing the quality of pile foundation with the acoustic transmission method, the acoustic time value read by the acoustic meter may be the acoustic time value when the ultrasonic wave meets the steel bar in the propagation path of the concrete. At this time, the measured sound time value may be part or all of the ultrasonic transmission in the reinforcement, and the propagation speed of the ultrasonic wave in the reinforcement is the propagation speed of the ultrasonic wave in the general concrete The speed is about 2 times. Therefore, it can be seen that in fact, the propagation time of ultrasonic wave in concrete is larger than that of actual ultrasonic wave in concrete. However, due to the above reasons, the calculated propagation speed of ultrasonic wave in concrete is too high, which will cause great deviation in the calculation of concrete strength. However, under normal circumstances, we usually tie the acoustic pipe to the inner side of the reinforcement cage, and the steel bar is rarely encountered in the process of ultrasonic propagation in concrete.
In the acoustic transmission method During pile foundation testing, The accepted wave amplitude is closely related to the quality of pile concrete, which is another important parameter to judge the quality of pile concrete. The amplitude of received acoustic wave is one of the indexes to characterize the energy attenuation degree of sound wave passing through concrete.
The amplitude of the received wave usually refers to the amplitude of the head wave, that is, the amplitude of the first half of the first periodic wave. The amplitude of received wave can reflect the strength of ultrasonic wave to a certain extent. The lower the amplitude of received wave is, the more attenuation of ultrasonic energy is in the process of ultrasonic propagation in concrete. When the ultrasonic wave propagates in the concrete with low strength and segregation, or when there are defects in the quality of pile foundation, the concrete will absorb more ultrasonic energy, and the attenuation of ultrasonic energy will become larger, which will lead to the decrease of received wave amplitude.
In the ultrasonic pile foundation detection, ultrasonic pulse has a variety of components, which is composed of a variety of frequency waves. In the process of ultrasonic propagation in concrete, the attenuation degree of high frequency wave and low frequency wave is different. In the process of ultrasonic wave propagation in concrete, the main frequency of receiving wave will decrease obviously because of the serious attenuation of ultrasonic wave and the loss of energy.
Waveform is an important index to judge the quality of pile foundation in ultrasonic testing. Whether the waveform is normal or not is often related to the abnormal sound velocity and amplitude. If the sound velocity value and amplitude value are abnormal, the waveform is abnormal, and the abnormal waveform is accompanied by the abnormal sound velocity value and amplitude value.
When ultrasonic wave encounters concrete defects in the process of propagation in concrete, due to the existence of acoustic interface with different impedance in the concrete, and on the concrete defect interface, ultrasonic will have various acoustic phenomena such as reflection, refraction, diffraction and so on. After reflection, refraction and diffraction, the propagation path of ultrasonic in concrete is different The time when they arrive at the receiving transducer is also different. The superposition of these ultrasonic waves will lead to waveform distortion. From this point of view, the waveform distortion can be used as a reference for judging the concrete defects of pile shaft to a certain extent.